diff --git a/backup.sh b/backup.sh
index e64e598..e6dc4c9 100755
--- a/backup.sh
+++ b/backup.sh
@@ -1,5 +1,7 @@
 #!/usr/bin/env sh
 
+# requires zfs, pv, ssh
+
 nodes_file="nodes.txt"
 ### nodes.txt
 # <origin host>   <user> <ssh port> <timezone> <compressed flag>
@@ -22,7 +24,7 @@ ZFS_SEND_OPTS="-pRLe"
 ZFS_SNAPSHOT_OPTS="-r"
 ZFS_RECV_OPTS="-vsu"
 SSH_SEND_OPTS="-Ti ~/.ssh/id_rsa"
-DEF_SPEED="20M"
+DEFAULT_SPEED="20M"
 
 read LOCAL_HOST LOCAL_USER LOCAL_PORT LOCAL_TZ SEND_COMPRESSED < $nodes_file
 
@@ -218,7 +220,7 @@ send_latest()
             debug "Processing $dset (remote $rset)."
             snap1=`get_latest_snapshot $dset`
             send_cmd="zfs send $ZFS_SEND_OPTS $dset@$snap1"
-            pv_cmd="pv -qL ${speed:-$DEF_SPEED}"
+            pv_cmd="pv -qL ${speed:-$DEFAULT_SPEED}"
             recv_cmd="zfs recv $ZFS_RECV_OPTS $zroot$rset"
             run_send_recv
         done
@@ -241,7 +243,7 @@ send_increment()
             snap1=`get_latest_snapshot`
             snap0=`get_latest_remote_snapshot`
             send_cmd="zfs send $ZFS_SEND_OPTS -i $dset@$snap0 $dset@$snap1"
-            pv_cmd="pv -qL ${speed:-$DEF_SPEED}"
+            pv_cmd="pv -qL ${speed:-$DEFAULT_SPEED}"
             recv_cmd="zfs recv $ZFS_RECV_OPTS $zroot$rset"
             run_send_recv
         done